MALACOLOGIA, 1981, 21(1-2): 209-262 DIFFERENT MODES OF EVOLUTION AND ADAPTIVE RADIATION IN THE POMATIOPSIDAE (PROSOBRANCHIA: MESOGASTROPODA) George M. Davis ^ Academy of Natural Sciences, Nineteenth and the Parkway, Philadelphia, PA 19103, U.S.A. ABSTRACT Two subfamilies of the Pomatiopsidae are shown to have different tempos and modes of evolution. Data for the Triculinae are not new but represent a synthesis of several data sets (Davis, 1979, 1980; Davis & Greer, 1980). Data for the Pomatiopsinae with emphasis on the Tomichia radiation of South Afhca are new. The distribution of modern pomatiopsid taxa is vicariant, a relict distribution with a secondary elaboration in Southeast Asia and the Far East extending to North America. There are eight pomatiopsine genera, one each in South Africa, South America, and Australia; one genus is found in an arc from western China to the Philippines and Sulawesi with taxa reaching Japan; two are endemic in Japan; one is found in fHanchuria, Japan, and western U.S.A.; one is endemic in the U.S.A. There are 16 triculine genera, all but one of which are located entirely in Southeast Asia or western China. Tricula extends in an arc from India through China to the Philippines and in an arc through Burma to Malaysia. The Triculinae have undergone an extraordinary endemic radiation in the Mekong River, yielding three tribes, 1 1 genera and over 90 species in a period of about 12 million years. This burst of cladogenesis was apparently driven by extrinsic processes correlated with the massive tectonics caused by the Himalayan orogeny that led to the formation of the major river systems of Southeast Asia, and western China. The morphological changes in the entirely aquatic group of snails that marked the entrance into various new adaptive zones involved a series of innova-tions in the female reproductive system, the male reproductive system posterior to the penis, and the central tooth of the radula. Bursts of speciation following each morphological innovation or series of correlated innovations yielded clusters of species that are considered discrete genera. The genera are separated by distinct gaps defined by morphological distances that are meas-ures of morphological changes indicative of entrances into new adaptive zones. Pomatiopsine taxa are aquatic, amphibious, or terrestrial. Modes of evolution in the Pomatiopsinae of the southern continents are in marked contrast to those in the Triculinae. In South Africa there are, at most, eight species of Tomichia with an evolutionary history of at least 80 million years. In Australia there are, at most, nine species of Coxiella. Tomictiia and Coxiella are very similar anatomically. No burst of cladogenesis or considerable speciation is seen. Species of Tomictiia do not differ very much in anatomy. The apparent low rate of speciation and lack of cladogenesis correlate with the lack of tectonic upheaval and gradual climatic changes since proto-Tomichia and proto-Cox/e//a were separated by the breakup of Gondwanaland. The limited Tomictiia radiation is apparently in response to increasing aridity spreading from west to east in South Africa since the breakup of Gondwanaland. Speciation has not involved morpho-logical modification but rather, adaptation to different ecological settings: freshwater streams, freshwater lakes, amphibious ecotones, temporary brackish water pools. Preadapted morpho-logical features for an amphibious existence were probably the large, powerful foot and the elongate spermathecal duct. The tempo of the Mekong River triculine evolution is rapid (R = about 0.40 contrasted with a slower rate (R = about 0.139) for the Tomichia radiation. The mode of triculine evolution is rapid, episodic speciation involving considerable morphological innovation and cladogenesis, all associated with extreme tectonism. The mode of Tomichia evolution involves a physiological radiation with low morphological diversity associated with gradual climatic change and general absence of tectonism. INTRODUCTION In considering tempos I am concerned with rates of cladogenesis, the number and extent Modes and tempos of evolution above the of adaptive radiations in phyletically allied species level are highly relevant topics for clades (per unit time), and the rate of extinc-contemporary students of biological evolution, tion of species and lineages. By extent of ""Supported by U.S. National Institutes of Health grant No. AM 1373. (209)
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